The present invention relates to a process for reducing the amount of alkyleneglycol diacrylate or alkyleneglycol dimethacrylate produced during the distillation of hydroxyalkyl acrylates or hydroxyalkyl methacrylates produced by the metal ion catalyzed esterification of an acrylic or methacrylic acid by an alkylene oxide. The term (meth)acrylate is used herein to describe both acrylate and methacrylate containing compounds or derivatives.
Hydroxyalkyl (meth)acrylates are typically produced by reaction of (meth)acrylic acid with an alkylene oxide in the presence of a metal ion catalyst. Metal ion catalysts include iron salts of organic acids such as iron (meth)acrylate, iron salts of inorganic acids such as ferric chloride, chromium compounds such as chromic or bichromic acid, Lewis acids such as aluminum chloride, as well as organic bases such as tertiary amines and quaternary ammonium salts. Of these metal ion catalysts, iron salts are the most common.
Purification of the hydroxyalkyl (meth)acrylate is generally accomplished by distillation from the crude reaction mixture. One of the major problems with this process is that the hydroxyalkyl (meth)acrylate itself is capable of further esterification with an additional (meth)acrylic acid molecule or transesterification with another hydroxyalkyl (meth)acrylate to produce an alkyleneglycol di(meth)acrylate (hereinafter referred to as "di(meth)acrylate"). The di(meth)acrylate is undesirable because it can act as a crosslinking agent in any subsequent polymerizations in which the hydroxyalkyl (meth)acrylate is a component. Furthermore, the di(meth)acrylate may increase the extent of unwanted polymerization during the distillation step resulting in increased amounts of tars or other unwanted solids in the still bottoms or on the equipment and a corresponding reduction in the yield of the desired hydroxyalkyl (meth)acrylate in the distillate. The extent of formation of this di(meth)acrylate is dependent on the type of catalyst used in the reaction, the concentration of the various reactants, products, or other additives, as well as the distillation conditions. An additional problem that may occur during distillation is that the reaction catalysts normally used often form a variety of side products including complexes with reactants and products in addition to the tars and unwanted solids. These complexes also often interfere with the distillation process.
U.S. Pat. No. 3,709,928 describes the use of polyalkyleneglycols, with higher boiling points than the desired hydroxyalkyl (meth)acrylates, to increase both the purity and overall yield of hydroxyalkyl (meth)acrylates produced by reacting an alkyl (meth)acrylic acid, or its sodium salt, and an alkylene oxide, or corresponding alkyl chlorohydrin, and isolating the product by distillation. The polyalkyleneglycols are reported to have two effects (1), they inhibit the formation of di(meth)acrylate, and (2), they prevent the catalyst used from forming an insoluble solid that interferes with the distillation. However, the exact mechanism of action of the polyalkyleneglycols is not disclosed.
U.S. Pat. No. 4,365,081 describes the preparation of 2-hydroxyalkyl methacrylate by the reaction of methacrylic acid with an alkylene oxide catalyzed by the ferric salt of a variety of carboxylic acids, including (meth)acrylic, fumaric, malic, benzoic, various phthalic, or salicylic. After distillation is complete, addition of water, acetic acid, salicylic acid, ethanolamines or methanol will decompose the distillation residue, which is characterized as an iron containing salt of complex structure. Furthermore, salicylic acid can be added anytime during the reaction or distillation to stabilize the distillation residue.
We approached the problem of di(meth)acrylate production by assuming that the reaction catalyst or catalyst complex also catalyzed the transesterification reaction of hydroxyalkyl (meth)acrylate with itself or its direct esterification with (meth)acrylic acid. Thus, the object of the invention was to discover an additive that would deactivate the catalyst, or catalyst byproducts, thereby avoiding the catalyzed transesterification or direct esterification with resulting di(meth)acrylate contamination of the hydroxyalkyl (meth)acrylate distillate. We have discovered that certain organic compounds have the appropriate mix of physicochemical properties such that they deactivate the catalyst, possibly via some type of sequestering process.
A variety of organic compounds have been found that deactivate the catalyst while minimizing contamination of the product. They share the following properties: a boiling point higher than the product hydroxyalkyl (meth)acrylate, functional groups capable of interacting with the metal cation, and solubility in the reaction mixture. When the boiling point is similar to or less than the product hydroxyalkyl (meth)acrylate, it is difficult to avoid contamination of the distillate with the additive.
Furthermore, the additive should preferably be a weaker acid than (meth)acrylic acid. We have found that when the additive's pK.sub.a is too low any unreacted (meth)acrylic acid that may have complexed with the metallic catalyst is liberated resulting in an increased (meth)acrylic acid level in the distillate. The mix of properties may be varied depending on the particular hydroxyalkyl (meth)acrylate being produced.
Additives found to substantially reduce di(meth)acrylate formation include, for example, the following: stearic acid, lauric acid, octanoic acid, p-toluic acid, phenylacetic acid, adipic acid, o-phthalic acid, succinic acid, citric acid, 2-hydroxyphenylacetic acid, phthalic anhydride, succinic anhydride, diglycolic anhydride, catechol, 3-methoxycatechol, 2-hydroxybenzyl alcohol, polyacrylic acids, and glycerol.
Preferred additives include the long chain fatty acids (e.g. stearic and lauric acids) and the diols (catechol, 3-methoxycatechol, and glycerol). Most preferred is catechol. The effective amount of additive is the amount required to reduce the di(meth)acrylate level in the distillate below that found in the absence of the additive. It may range from about 0.1 to about 5 weight percent. However, from about 0.5 to about 2.0 weight percent is preferred, and from about 0.75 to about 1.0 weight percent is most preferred.